1. Field of the Invention
The present invention relates generally to a vibratory motion restrainer, and more specifically to a mechanism for reducing or depressing externally applied oscillations to a rack-mounted card cage by way of a rack, which cage accommodates a plurality of printed circuit boards.
2. Description of Related Art
As is known in the art, groups of printed circuit boards are interconnected by placing them in card cages (also known as shelves or sub-sacks) which include hardware for housing the printed circuit boards.
Prior to turning to the present invention, it is deemed preferable to briefly describe, with reference to FIGS. 1 and 2, a conventional rack and card cages, to which the present invention is applicable.
FIG. 1 is a perspective view schematically showing part of rack 10 and one card cage 12 to be housed therein. As shown, the rack 10 comprises four frames 14a-14d vertically provided at four corners of the rack 10, two side panels 16a and 16b secured to the frames 14a-14d, and a back panel 16c fixed to the frames 14b and 14c. The rack 10 further comprises a plurality of pairs of card cage ledges (shelves), each pair of which is used to support the card cage at left and right bottoms thereof. More specifically, FIG. 1 shows that the card cage 12 is supported by a pair of cage ledges (only one ledge 20a is partially shown). On the other hand, another pair of cage ledges 22a-22b are secured to the opposite inner sides of the rack 10 so as to support another card cage (not shown) to be installed into the rack 10 above the cage 12.
The card cage 12 comprises four frames 24 (one frame is hidden in FIG. 1) transversely positioned to tops and bottoms of front and rear portions thereof. The cage 12 further comprises two side panels 26a-26b respectively secured to the right and left sides of the cage 12, and a rear panel 26c fixed to the rear portion thereof. The side panels 26a-26b respectively have front edges 27a-27b which are respectively used to detachably fix the card cage 12 to the rack frames 14a-14b. Further, a plurality of guide rails 28 are mounted at the tops and bottoms of the four frames 24 at regular intervals between the side panels 26a-26b. 
A plurality of card units 30 are detachably inserted into the corresponding guide rails 28, leaving a predetermined space. between the adjacent ones. The removing and extracting of the card units is for maintenance, upgrade or other purposes. Each card unit 30 is provided with a front panel 32, a printed circuit board 34, a unit connector 36 mating with a corresponding connector 38 (FIG. 2) provided on the inner wall of the rear panel 16c. Further, each unit 30 comprises a pair of ejectors 40 in the vicinity of top and bottom portions of the front panel 32. It is preferable to provide the ejector 40 with a suitable unit locking mechanism so as to prevent unintentional removal of the card unit 30 from the cage 12. It is to be noted that FIG. 1 omits a plurality of resilient supporting members provided between the rack 10 and the card cage 12, which will be referred to in FIG. 2.
FIG. 2 is a sectional plan view taken along a section line Axe2x80x94A of FIG. 1. In order to simplify the drawing, the front panel 32, the guide rails 28, etc. are omitted in that they are impertinent to the present invention.
As shown in FIG. 2. the cage 12, which has accommodated the multiple printed circuit boards 34, is housed in the rack 10 (FIG. 1), in the case of which each card unit connector 36 is electrically coupled to the corresponding cage connector 38. The card cage 12 is detachably secured to the rack frames 14a and 14d respectively at the elongated front edges 27a-27b using a suitable fastening member 50 such as a screw. On the other hand, resilient support members 52 are respectively fixed to the rack frames 14b and 14c using screws 54 at the rear portions thereof. The above-mentioned support members 52, however, may be welded to the corresponding rack frames 14b-14c in place of the screws 54. When the card cage 12 is completely inserted into the rack 10 as illustrated, the resilient support members 52 press the rear side portions of the cage 12, thereby rigidly supporting the same in the transverse direction, viz., a direction normal to that along which the printed circuit board 34 are inserted. The resiliently supporting technology, as just mentioned above, is disclosed in Japanese Laid-open Patent Application No. 10-93264. As an alternative, the resilient support members 52 may be fixed, using screws (for example), to the rear sides of the card cage. 12.
When the rack 10 is transported by land, sea, or air while accommodating the card units 30 (viz., printed circuit boards 34)within the cage(s) 12, the rack 10 is typically subject to vibrations or oscillations with a variety of frequencies and magnitudes. Further, even after the rack 10 has been set, the rack 10 may be subject to vibrations with low frequency and large amplitude due to earthquake (for example). Therefore, if the cage 12 is secured to the rack 10 using the rigid supporting members 52 at the rear portions, and fixed to the rack frames 14a-14d at the front edges 27a-27b, it is not expected to effectively reducing the externally applied vibrations with high frequencies and relatively low amplitude. On the other hand, if the supporting members 52 are made of material with high elasticity so as to high frequencies and low amplitude vibrations, it is in turn unable to effectively absorb the vibrations with low frequencies and large amplitudes.
If the vibrations applied to the card cage 12 coincides with the characteristic (natural) frequency of the printed circuit board, the electronic components may be damaged due to sympathetic vibrations. Further, in such a case, the adjacent card units may collide with each other, resulting in damages of the card units and/or the printed circuit boards mounted thereon.
It is therefore an object of the present invention to provide a mechanism for reducing vibratory motions applied to the rack-mounted card cage by changing a spring constant of a resilient member in response to the magnitude and frequencies of the vibration.
In brief, the object is achieved by the techniques wherein a mechanism for reducing vibratory motion applied to a card cage mounted in a rack is provided. The cage accommodates a plurality of printed circuit boards arranged in parallel at regular intervals. The mechanism comprises dampers provided between the rack and the card cage. Each of the dampers is responsive to external vibrations applied to the rack in a transverse direction relative to a direction along which the plurality of printed circuit boards are arranged, and changes damping characteristics thereof in two stages depending on strengths and frequencies of the external vibrations.
One aspect of the present invention resides in a mechanism for reducing vibratory motion applied to a card cage mounted in a rack, the card cage accommodating a plurality of printed circuit boards arranged in parallel at regular intervals, the mechanism comprising: damping means, provided between the rack and the card cage, for responding to external vibrations applied to the rack in a transverse direction relative to a direction along which the plurality of printed circuit boards are arranged, and changing damping characteristics thereof in two stages depending on strengths and frequencies of the external vibrations.